Short-chain fatty acids affect cell-association and invasion of HEp-2 cells by Salmonella typhimurium

Butyrate affects bacterial virulence: a new perspective on preventing enteric bacterial pathogen invasion

Enteric bacterial pathogens are a major threat to intestinal health. With the widespread use of antibiotics, bacterial resistance has become a problem, and there is an urgent need for a new treatment to reduce dependence on antibiotics. Butyrate can control enteric bacterial pathogens by regulating the expression of their virulence genes, promoting the posttranslational modification of their proteins, maintaining an anaerobic environment, regulating the host immune system and strengthening the intestinal mucosal barrier. Here, this review describes the mechanisms by which butyrate regulates the pathogenicity of enteric bacterial pathogens from various perspectives and discusses the prospects and limitations of butyrate as a new option for the control of pathogenic bacteria.

The in Vitro Fermentation of Cordyceps militaris Polysaccharides Changed the Simulated Gut Condition and Influenced Gut Bacterial Motility and Translocation

The motility ability of intestinal lipopolysaccharide (LPS)-producing bacteria determines their translocation to the enterohepatic circulation and works as an infectious complication. In this study, the health effects of Cordyceps militaris polysaccharides (CMPs) were re-evaluated based on whether these polysaccharides could affect the motility of gut commensal LPS-producing bacteria and impede their translocation. The results showed that CMP-m fermentation in the gut could change the chemical environment, leading to a decrease in velocity and a shift in the motility pattern. Further study suggested that detachment/fragmentation of flagella, decreased motor forces, and changed chemical conditions might account for this weakened motility. The adhesion and invasion abilities of gut bacteria were also reduced, with lower expression of virulence-related genes. These results indicated that the health regulation effects of CMP-m might be through decreasing the motility of LPS-producing bacteria, hindering their translocation and therefore reducing the LPS level in the enterohepatic circulation.

In vitro evaluation of sodium butyrate on the growth of three Salmonella serovars derived from pigs at a mild acidic pH value

Foodborne zoonotic diseases can be transferred into the food chain at the stage of livestock farming. As an emerging public health challenge, practicable reduction measures in porcine health management for Salmonella are constantly being investigated. This in vitro study aimed to determine the influence of six different sodium butyrate (SB) concentrations (0, 5, 10, 20, 40, and 80 mM) on the growth of three different Salmonella enterica serovars at a constant pH value of 6.0, corresponding to conditions in the pig's hindgut. S. Derby and S. Typhimurium, isolated from a pig farm, and S. Typhimurium DSM 19587, which served as control, were used. Broth microdilution assay was applied to record Salmonella growth in the presence of different SB-concentrations over six different incubation periods (0, 1, 2, 4, 6, and 24 h). Results were quantified in the log colony-forming units (log10 CFU/mL). For 1 h incubation, the addition of SB showed no significant differences in the range of initial Salmonella dose of about 5.7 log10 between concentrations (0-80 mM, 5.26 ± 0.10-5.60 ± 0.07 log10, p > 0.05). After 6 h, for SB addition, the range of Salmonella counts was significantly lower compared to no addition of SB (5-80 mM, p < 0.05), 6.78 ± 0.84-7.90 ± 0.10 log10 for 5 mM, and 7.53 ± 0.04-8.71 ± 0.22 log10 for 0 mM. Moreover, for SB concentrations of 40 and 80 mM, no difference in the range of Salmonella counts over 6 h was obtained (5.23 ± 0.11-5.38 ± 0.05 log10, p > 0.05), and minor Salmonella growth was recorded at the earliest after 24 h incubation. Growth rates for varying SB concentrations and incubation times were confirmed in a similar manner for the three serovars. Obtained results suggest that increasing SB concentrations suppress Salmonella growth for concentrations of 5-20 mM over a 6 h incubation period and for 40 and 80 mM over a 24 h incubation period. When transferring these in vitro findings to the porcine organism, it may be assumed that Salmonella reduction can be achieved by increased butyrate content in the chyme of the large intestine.

Dual role of microbiota-derived short-chain fatty acids on host and pathogen

A growing body of documents shows microbiota produce metabolites such as short-chain fatty acids (SCFAs) as crucial executors of diet-based microbial influence the host and bacterial pathogens. The production of SCFAs depends on the metabolic activity of intestinal microflora and is also affected by dietary changes. SCFAs play important roles in maintaining colonic health as an energy source, as a regulator of gene expression and cell differentiation, and as an anti-inflammatory agent. Additionally, the regulated expression of virulence genes is critical for successful infection by an intestinal pathogen. Bacteria rely on sensing environmental signals to find preferable niches and reach the infectious state. This review will present data supporting the diverse functional roles of microbiota-derived butyrate, propionate, and acetate on host cellular activities such as immune modulation, energy metabolism, nervous system, inflammation, cellular differentiation, and anti-tumor effects, among others. On the other hand, we will discuss and summarize data about the role of these SCFAs on the virulence factor of bacterial pathogens. In this regard, receptors and signaling routes for SCFAs metabolites in host and pathogens will be introduced.

Short-Chain Fatty Acids as a Potential Treatment for Infections: a Closer Look at the Lungs

Short-chain fatty acids (SCFAs) are the main metabolites produced by the gut microbiota via the fermentation of complex carbohydrates and fibers. Evidence suggests that SCFAs play a role in the control of infections through direct action both on microorganisms and on host signaling. This review summarizes the main microbicidal effects of SCFAs and discusses studies highlighting the effect of SCFAs in the virulence and viability of microorganisms. We also describe the diverse and complex modes of action of the SCFAs on the immune system in the face of infections with a specific focus on bacterial and viral respiratory infections. A growing body of evidence suggests that SCFAs protect against lung infections. Finally, we present potential strategies that may be leveraged to exploit the biological properties of SCFAs for increasing effectiveness and optimizing patient benefits.

Recent advances in the therapeutic application of short-chain fatty acids (SCFAs): An updated review

Over the past decade, the gut microbiota has emerged as an important frontier in understanding the human body's homeostasis and the development of diseases. Gut flora in human beings regulates various metabolic functionalities, including enzymes, amino acid synthesis, bio-transformation of bile acid, fermentation of non-digestible carbohydrates (NDCs), generation of indoles and polyamines (PAs), and production of short-chain fatty acids (SCFAs). Among all the metabolites produced by gut microbiota, SCFAs, the final product of fermentation of dietary fibers by gut microbiota, receive lots of attention from scientists due to their pharmacological and physiological characteristics. However, the molecular mechanisms underlying the role of SCFAs in the interaction between diet, gut microbiota, and host energy metabolism is still needed in-depth research. This review highlights the recent biotechnological advances in applying SCFAs as important metabolites to treat various diseases and maintain colonic health.

Deletion of from Streptococcus pyogenes. Results in Hypervirulence in a Mouse Model of Sepsis and is LuxS Independent

Group A Streptococcus (GAS) is a Gram-positive human pathogen that causes a variety of diseases ranging from pharyngitis to life-threatening streptococcal toxic shock syndrome. Recently, several global gene expression analyses have yielded extensive new information regarding the regulation of genes encoding known and putative virulence factors in GAS. A microarray analysis found that transcription of the GAS gene M5005_Spy_1343 was significantly increased in response to interaction with human polymorphonuclear leukocytes. M5005_Spy_1343 is predicted to encode a member of the LysR family of transcriptional regulators and is located upstream of a putative operon containing six genes. Five of these genes have sequence similarity to genes involved in short-chain fatty acid metabolism, whereas the sixth gene (luxS) is found in many bacterial species and is involved in quorum sensing. Unexpectedly, inactivation of the M5005_Spy_1343 gene resulted in hypervirulence in an intraperitoneal mouse model of infection. Increased virulence was not due to changes in luxS gene expression. We postulate that short-chain fatty acid metabolism is involved in GAS pathogenesis.

Reduced particle size wheat bran is butyrogenic and lowers Salmonella colonization, when added to poultry feed

Feed additives, including prebiotics, are commonly used alternatives to antimicrobial growth promoters to improve gut health and performance in broilers. Wheat bran is a highly concentrated source of (in)soluble fiber which is partly degraded by the gut microbiota. The aim of the present study was to investigate the potential of wheat bran as such to reduce colonization of the cecum and shedding of Salmonella bacteria in vivo. Also, the effect of particle size was evaluated. Bran with an average reduced particle size of 280μm decreased levels of cecal Salmonella colonization and shedding shortly after infection when compared to control groups and groups receiving bran with larger particle sizes. In vitro fermentation experiments revealed that bran with smaller particle size was fermented more efficiently, with a significantly higher production of butyric and propionic acid, compared to the control fermentation and fermentation of a larger fraction. Fermentation products derived from bran with an average particle size of 280μm downregulated the expression of hilA, an important invasion-related gene of Salmonella. This downregulation was reflected in an actual lowered invasive potential when Salmonella bacteria were pretreated with the fermentation products derived from the smaller bran fraction. These data suggest that wheat bran with reduced particle size can be a suitable feed additive to help control Salmonella infections in broilers. The mechanism of action most probably relies on a more efficient fermentation of this bran fraction and the consequent increased production of short chain fatty acids (SCFA). Among these SCFA, butyric and propionic acid are known to reduce the invasion potential of Salmonella bacteria.

Feed presentation options in Swine early fattening mitigates Salmonella shedding and specifically modulates the faecal microbiota

AIMS

The object of this study was to determine the impact of only modifying the processing and/or particle size of pig feed on Salmonella shedding and faecal microbiota.

METHODS AND RESULTS

Pigs were fed a diet that varied only by their processing (pellet or mash) and their particle size (500, 750 or 1250 μm) for 21 days. Salmonella detection in faeces and seroconversion were determined. Faecal microbiota was assessed by Ion Torrent amplicon sequencing and real-time PCR. Significantly fewer pigs (P < 0·05) shed Salmonella in the groups fed mash 500 (1) and mash or pellet 1250 (5 each) compared to the commercial reference group (15) fed pellet 500. Both mash processing and large particle size raised the proportion and number of bacteria from the Bifidobacterium genus in the faecal microbiota of the pigs. Thirteen other taxa significantly varied (P < 0·0005) with feed presentation.

CONCLUSION

Mash processing and/or large particle size in pig feed reduces Salmonella shedding prevalence and promotes beneficial populations of digestive microbiota.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is the first to demonstrate a difference in Salmonella shedding through only modifying pig feed presentation and is the first to extensively describe modifications of faecal microbiota.

Application of Molecular Approaches for Understanding Foodborne Salmonella Establishment in Poultry Production

Salmonellosis in the United States is one of the most costly foodborne diseases. Given that Salmonella can originate from a wide variety of environments, reduction of this organism at all stages of poultry production is critical. Salmonella species can encounter various environmental stress conditions which can dramatically influence their survival and colonization. Current knowledge of Salmonella species metabolism and physiology in relation to colonization is traditionally based on studies conducted primarily with tissue culture and animal infection models. Consequently, while there is some information about environmental signals that control Salmonella growth and colonization, much still remains unknown. Genetic tools for comprehensive functional genomic analysis of Salmonella offer new opportunities for not only achieving a better understanding of Salmonella pathogens but also designing more effective intervention strategies. Now the function(s) of each single gene in the Salmonella genome can be directly assessed and previously unknown genetic factors that are required for Salmonella growth and survival in the poultry production cycle can be elucidated. In particular, delineating the host-pathogen relationships involving Salmonella is becoming very helpful for identifying optimal targeted gene mutagenesis strategies to generate improved vaccine strains. This represents an opportunity for development of novel vaccine approaches for limiting Salmonella establishment in early phases of poultry production. In this review, an overview of Salmonella issues in poultry, a general description of functional genomic technologies, and their specific application to poultry vaccine developments are discussed.

Salmonella pathogenicity and host adaptation in chicken-associated serovars

Enteric pathogens such as Salmonella enterica cause significant morbidity and mortality. S. enterica serovars are a diverse group of pathogens that have evolved to survive in a wide range of environments and across multiple hosts. S. enterica serovars such as S. Typhi, S. Dublin, and S. Gallinarum have a restricted host range, in which they are typically associated with one or a few host species, while S. Enteritidis and S. Typhimurium have broad host ranges. This review examines how S. enterica has evolved through adaptation to different host environments, especially as related to the chicken host, and continues to be an important human pathogen. Several factors impact host range, and these include the acquisition of genes via horizontal gene transfer with plasmids, transposons, and phages, which can potentially expand host range, and the loss of genes or their function, which would reduce the range of hosts that the organism can infect. S. Gallinarum, with a limited host range, has a large number of pseudogenes in its genome compared to broader-host-range serovars. S. enterica serovars such as S. Kentucky and S. Heidelberg also often have plasmids that may help them colonize poultry more efficiently. The ability to colonize different hosts also involves interactions with the host's immune system and commensal organisms that are present. Thus, the factors that impact the ability of Salmonella to colonize a particular host species, such as chickens, are complex and multifactorial, involving the host, the pathogen, and extrinsic pressures. It is the interplay of these factors which leads to the differences in host ranges that we observe today.

The intestinal fatty acid propionate inhibits Salmonella invasion through the post-translational control of HilD

To cause disease, Salmonella must invade the intestinal epithelium employing genes encoded within Salmonella Pathogenicity Island 1 (SPI1). We show here that propionate, a fatty acid abundant in the intestine of animals, repressed SPI1 at physiologically relevant concentration and pH, reducing expression of SPI1 transcriptional regulators and consequently decreasing expression and secretion of effector proteins, leading to reduced bacterial penetration of cultured epithelial cells. Essential to repression was hilD, which occupies the apex of the regulatory cascade within SPI1, as loss of only this gene among those of the regulon prevented repression of SPI1 transcription by propionate. Regulation through hilD, however, was achieved through the control of neither transcription nor translation. Instead, growth of Salmonella in propionate significantly reduced the stability of HilD. Extending protein half-life using a Lon protease mutant demonstrated that protein stability itself did not dictate the effects of propionate and suggested modification of HilD with subsequent degradation as the means of action. Furthermore, repression was significantly lessened in a mutant unable to produce propionyl-CoA, while further metabolism of propionyl-CoA appeared not to be required. These results suggest a mechanism of control of Salmonella virulence in which HilD is post-translationally modified using the high-energy intermediate propionyl-CoA.

Regulation of bacterial pathogenesis by intestinal short-chain Fatty acids

The human gut microbiota is inextricably linked to health and disease. One important function of the commensal organisms living in the intestine is to provide colonization resistance against invading enteric pathogens. Because of the complex nature of the interaction between the microbiota and its host, multiple mechanisms likely contribute to resistance. In this review, we dissect the biological role of short-chain fatty acids (SCFA), which are fermentation end products of the intestinal microbiota, in host-pathogen interactions. SCFA exert an extensive influence on host physiology through nutritional, regulatory, and immunomodulatory functions and can also affect bacterial fitness as a form of acid stress. Moreover, SCFA act as a signal for virulence gene regulation in common enteric pathogens. Taken together, these studies highlight the importance of the chemical environment where the biology of the host, the microbiota, and the pathogen intersects, which provides a basis for designing effective infection prevention and control.

Mitsuokella jalaludinii inhibits growth of Salmonella enterica serovar Typhimurium

Salmonella continues to be a significant human health threat, and the objective of this study was to identify microorganisms with the potential to improve porcine food-safety through their antagonism of Salmonella. Anaerobic culture supernatants of 973 bacterial isolates from the gastrointestinal tract and feces of swine were screened for their capacity to inhibit the growth of Salmonella enterica serovar Typhimurium. Growth inhibition of 1000-fold or greater was observed from 16 isolates, and 16S rRNA sequencing identified the isolates as members of the genera Mitsuokella, Escherichia/Shigella, Anaerovibrio, Selenomonas, and Streptococcus. Four isolates were identified as Mitsuokella jalaludinii, and the mechanism of Salmonella Typhimurium growth inhibition by M. jalaludinii was further investigated. M. jalaludinii stationary phase culture supernatants were observed to significantly inhibit growth, and featured the production of lactic, succinic, and acetic acids. Aerobic and anaerobic S. Typhimurium growth was restored when the pH of the culture supernatants (pH 4.6) was increased to pH 6.8. However, S. Typhimurium growth in fermentation acid-free media was the same at pH 4.6 and pH 6.8 - indicating a synergistic effect between fermentation acid production and low pH as the cause of S. Typhimurium growth inhibition. Furthermore, exposure of S. Typhimurium to M. jalaludinii culture supernatants inhibited Salmonella invasion of HEp-2 cells by 10-fold. The results identify M. jalaludinii as a possible inhibitor of Salmonella growth and invasion in swine, and thus a potential probiotic capable of improving food safety.

Protective effect of probiotics on Salmonella infectivity assessed with combined in vitro gut fermentation-cellular models

Background

Accurate assessment of probiotics with targeted anti-Salmonella activity requires suitable models accounting for both, microbe-microbe and host-microbe interactions in gut environments. Here we report the combination of two original in vitro intestinal models closely mimicking the complex in vivo conditions of the large intestine. Effluents from continuous in vitro three-stage fermentation colonic models of Salmonella Typhimurium infection inoculated with immobilized child microbiota and Salmonella were directly applied to confluent mucus-secreting HT29-MTX cell layers. The effects of Salmonella, addition of two bacteriocinogenic strains, Bifidobacterium thermophilum RBL67 (thermophilicin B67) and Escherichia coli L1000 (microcin B17), and inulin were tested on Salmonella growth and interactions with epithelial cell layers. Salmonella adhesion and invasion were investigated and epithelial integrity assessed by transepithelial electrical resistance (TER) measurements and confocal microscopy observation. Data from complex effluents were compared with pure Salmonella cultures.

Results

Salmonella in effluents of all reactors of the colonic fermentation model stabilized at mean values of 5.3 ± 0.8 log₁₀ cfu/ml effluent. Invasion of cell-associated Salmonella was up to 50-fold lower in complex reactor samples compared to pure Salmonella cultures. It further depended on environmental factors, with 0.2 ± 0.1% being measured with proximal, 0.6 ± 0.2% with transverse and 1.3 ± 0.7% with distal reactor effluents, accompanied by a similar high decrease of TER across cell monolayers (minus 45%) and disruption of tight junctions. Subsequent addition of E. coli L1000 stimulated Salmonella growth (6.4 ± 0.6 log₁₀ cfu/ml effluent of all 3 reactors) and further decreased TER, but led to 10-fold decreased invasion efficiency when tested with distal reactor samples. In contrast, presence of B. thermophilum RBL67 revealed a protective effect on epithelial integrity compared to previous E. coli L1000 periods, as reflected by a significant mean increase of TER by 58% in all reactors. Inulin addition enhanced Salmonella growth and invasion when tested with distal and proximal reactor samples, respectively, but induced a limited decrease of TER (minus 18%) in all reactors.

Conclusions

Our results highlight the benefits of combining suitable cellular and colonic fermentation models to assess strain-specific first-level host protection properties of probiotics during Salmonella infection, providing an efficient system biology tool for preclinical development of new antimicrobials.

Foodborne Salmonella ecology in the avian gastrointestinal tract

Foodborne Salmonella continues to be a major cause of salmonellosis with Salmonella Enteritidis and S. Typhimurium considered to be responsible for most of the infections. Investigation of outbreaks and sporadic cases has indicated that food vehicles such as poultry and poultry by-products including raw and uncooked eggs are among the most common sources of Salmonella infections. The dissemination and infection of the avian intestinal tract remain somewhat unclear. In vitro incubation of Salmonella with mammalian tissue culture cells has shown that invasion into epithelial cells is complex and involves several genetic loci and host factors. Several genes are required for the intestinal phase of Salmonella invasion and are located on Salmonella pathogenicity island 1 (SPI 1). Salmonella pathogenesis in the gastrointestinal (GI) tract and the effects of environmental stimuli on gene expression influence bacterial colonization and invasion. Furthermore, significant parameters of Salmonella including growth physiology, nutrient availability, pH, and energy status are considered contributing factors in the GI tract ecology. Approaches for limiting Salmonella colonization have been primarily based on the microbial ecology of the intestinal tract. In vitro studies have shown that the toxic effects of short chain fatty acids (SCFA) to some Enterobacteriaceae, including Salmonella, have resulted in a reduction in population. In addition, it has been established that native intestinal microorganisms such as Lactobacilli provide protective mechanisms against Salmonella in the ceca. A clear understanding of the key factors involved in Salmonella colonization in the avian GI tract has the potential to lead to better approach for more effective control of this foodborne pathogen.

The use of organic acids to combatSalmonellain poultry: a mechanistic explanation of the efficacy

Salmonella is a human pathogen that is commonly found in poultry products. It is possible to decrease chicken carcass and egg contaminations by adding organic acids to the feed or drinking water at appropriate times. Medium-chain fatty acids are more antibacterial against Salmonella than short-chain fatty acids. The antibacterial effect of these acids is species specific. Bacteria that are unable to decrease intracellular pH accumulate organic acid anions in accordance with the pH gradient across their cell membranes. The short-chain fatty acid butyrate specifically down-regulates expression of invasion genes in Salmonella spp. at low doses. Also medium-chain fatty acids and propionate decrease the ability of Salmonella spp. to invade epithelial cells, in contrast to acetic acid. Because not all bacteria are affected in a similar fashion by organic acids, it may be possible to use probiotic and prebiotic bacteria to achieve beneficial effects. If diets can be designed to stimulate organic acid production in the caecum, it may be possible to control Salmonella spp. via even easier and more cost-effective measures, compared with addition of acids to feed or drinking water.

Kinetic analysis of the antibacterial activity of probiotic lactobacilli towards Salmonella enterica serovar Typhimurium reveals a role for lactic acid and other inhibitory compounds

Six Lactobacillus strains including commercial probiotic ones (L. acidophilus IBB 801, L. amylovorus DCE 471, L. casei Shirota, L. johnsonii La1, L. plantarum ACA-DC 287 and L. rhamnosus GG) were investigated, through batch fermentations under controlled conditions, for their capacity to inhibit Salmonella enterica serovar Typhimurium SL1344. All lactobacilli displayed strong antibacterial activity toward this Gram-negative pathogen and significantly inhibited invasion of the pathogen into cultured human enterocyte-like Caco-2/TC7 cells. By studying the production kinetics of antibacterial activity and applying the appropriate acid and pH control samples during a killing assay, we were able to distinguish between the effect of lactic acid and other inhibitory compounds produced. The antibacterial activity of L. acidophilus IBB 801, L. amylovorus DCE 471, L. casei Shirota and L. rhamnosus GG was solely due to the production of lactic acid. The antibacterial activity of L. johnsonii La1 and L. plantarum ACA-DC 287 was due to the production of lactic acid and (an) unknown inhibitory substance(s). The latter was (were) only active in the presence of lactic acid. In addition, the lactic acid produced was responsible for significant inhibitory activity upon invasion of Salmonella into Caco-2/TC7 cells.

Salmonella stress management and its relevance to behaviour during intestinal colonisation and infection

The enteric pathogen Salmonella enterica is exposed to a number of stressful environments during its life cycle within and outside its various hosts. During intestinal colonisation Salmonella is successively exposed to acid pH in the stomach, to the detergent-like activity of bile, to decreasing oxygen supply, to the presence of multiple metabolites produced by the normal gut microflora and finally it is exposed to cationic antimicrobial peptides present on the surface of epithelial cells. There are four major regulators controlling relevant stress responses in Salmonella, namely RpoS, PhoPQ, Fur and OmpR/EnvZ. Except for Fur, inactivation of genes encoding the other stress regulators results in attenuated virulence and such mutants can therefore be considered as vaccine candidates. In contrast, a decrease in oxygen supply monitored by Fnr and ArcAB, or oxidative stress controlled by OxyR and SoxRS is not regarded as a stress associated with host colonisation since inactivation of either of these systems does not result in reductions in colonisation. The role of quorum-sensing through luxS and sdiA is also considered as a regulator of virulence and colonisation.

Assessment of the effects of Nurmi-type cultures and a defined probiotic preparation on a Salmonella typhimurium 29E challenge in vivo

The effects of treatment with an undefined commercial Nurmi-type culture (NTC), cultured cecal contents, and a dual-strain probiotic, containing Enterococcus faecalis and Pediococcus pentosaceus, on Salmonella Typhimurium colonization were evaluated in a specific-pathogen-free bird model. Two sets of trials were performed, and each study was arranged as a randomized complete block design with three treatments. Treatments consisted of (i) control, (ii) commercial NTC, and (iii) cultured cecal contents in the first set of trials and (i) control, (ii) defined probiotic, and (iii) cultured cecal contents in the second set. On day 1, birds were administered 1.2 x 10(7) CFU of the appropriate treatment by oral gavage. On day 3, all birds were challenged with 1 x 10(6) CFU of Salmonella Typhimurium 29E (nalidixic acid resistant). Chicks were asphyxiated with argon gas on day 10, and ceca were aseptically removed. Salmonella Typhimurium counts (CFU per milliliter of cecal contents) were determined on brilliant green agar containing 30 mg of nalidixic acid per liter, and CFU counts were log transformed prior to analysis. Cecal pH and volatile fatty acid concentrations were also determined. Data were analyzed by one-way analysis of variance, and means were compared by Tukey's pairwise analysis. Commercial NTC and cultured cecal contents treatments resulted in a significant decrease (P < or = 0.05) in Salmonella Typhimurium 29E colonization, with the NTC offering a higher level of protection. In the second set of trials, the defined probiotic tended to reduce colonization by Salmonella Typhimurium (P = 0.07), while chicks treated with cultured cecal contents displayed a significant decrease (P = 0.03) when compared to the negative control. No significant change was observed in cecal pH or in acetate and propionate concentrations; however, a significant increase in butyrate concentrations in both the cultured cecal contents and defined probiotic treatment groups was observed when compared to the control birds. These observations suggest that defined cultures are less effective Salmonella control agents than are preparations generated from the complete cecal microflora.

The acetate switch

To succeed, many cells must alternate between life-styles that permit rapid growth in the presence of abundant nutrients and ones that enhance survival in the absence of those nutrients. One such change in life-style, the "acetate switch," occurs as cells deplete their environment of acetate-producing carbon sources and begin to rely on their ability to scavenge for acetate. This review explains why, when, and how cells excrete or dissimilate acetate. The central components of the "switch" (phosphotransacetylase [PTA], acetate kinase [ACK], and AMP-forming acetyl coenzyme A synthetase [AMP-ACS]) and the behavior of cells that lack these components are introduced. Acetyl phosphate (acetyl approximately P), the high-energy intermediate of acetate dissimilation, is discussed, and conditions that influence its intracellular concentration are described. Evidence is provided that acetyl approximately P influences cellular processes from organelle biogenesis to cell cycle regulation and from biofilm development to pathogenesis. The merits of each mechanism proposed to explain the interaction of acetyl approximately P with two-component signal transduction pathways are addressed. A short list of enzymes that generate acetyl approximately P by PTA-ACKA-independent mechanisms is introduced and discussed briefly. Attention is then directed to the mechanisms used by cells to "flip the switch," the induction and activation of the acetate-scavenging AMP-ACS. First, evidence is presented that nucleoid proteins orchestrate a progression of distinct nucleoprotein complexes to ensure proper transcription of its gene. Next, the way in which cells regulate AMP-ACS activity through reversible acetylation is described. Finally, the "acetate switch" as it exists in selected eubacteria, archaea, and eukaryotes, including humans, is described.

Interactions of butyric acid- and acetic acid-treated Salmonella with chicken primary cecal epithelial cells in vitro

In vitro studies of the interaction between pathogenic bacteria and the chicken intestinal epithelium are hampered by the lack of a host- and tissue-specific in vitro model. Therefore, a reproducible method for isolation and cultivation of chicken primary cecal epithelial cells was developed. Cecal crypts were isolated and cultured in vitro to form a semiconfluent layer of epithelial cells. Incubation of Salmonella enteritidis with these cells resulted in invasion. Pretreatment of the Salmonella bacteria with butyric acid resulted in a significant decrease of invasion of the bacteria in the chicken cecal epithelial cells, whereas pretreatment with acetic acid increased invasiveness. These interactions of S. enteritidis with primary chicken cecal epithelial cells were similar to the interactions with other epithelial cell types.

Medium-chain fatty acids decrease colonization and invasion through hilA suppression shortly after infection of chickens with Salmonella enterica serovar Enteritidis

The most common source of Salmonella infections in humans is food of poultry origin. Salmonella enterica serovar Enteritidis has a particular affinity for the contamination of the egg supply. In this study, the medium-chain fatty acids (MCFA), caproic, caprylic, and capric acid, were evaluated for the control of Salmonella serovar Enteritidis in chickens. All MCFA were growth inhibiting at low concentrations in vitro, with caproic acid being the most potent. Contact of Salmonella serovar Enteritidis with low concentrations of MCFA decreased invasion in the intestinal epithelial cell line T84. By using transcriptional fusions between the promoter of the regulatory gene of the Salmonella pathogenicity island I, hilA, and luxCDABE genes, it was shown that all MCFA decreased the expression of hilA, a key regulator related to the invasive capacity of Salmonella. The addition of caproic acid (3 g/kg of feed) to the feed of chicks led to a significant decrease in the level of colonization of ceca and internal organs by Salmonella serovar Enteritidis at 3 days after infection of 5-day-old chicks. These results suggest that MCFA have a synergistic ability to suppress the expression of the genes required for invasion and to reduce the numbers of bacteria in vivo. Thus, MCFA are potentially useful products for reducing the level of colonization of chicks and could ultimately aid in the reduction of the number of contaminated eggs in the food supply.

Perspectives on the use of organic acids and short chain fatty acids as antimicrobials

Organic acids have a long history of being utilized as food additives and preservatives for preventing food deterioration and extending the shelf life of perishable food ingredients. Specific organic acids have also been used to control microbial contamination and dissemination of foodborne pathogens in preharvest and postharvest food production and processing. The antibacterial mechanism(s) for organic acids are not fully understood, and activity may vary depending on physiological status of the organism and the physicochemical characteristics of the external environment. An emerging potential problem is that organic acids have been observed to enhance survivability of acid sensitive pathogens exposed to low pH by induction of an acid tolerance response and that acid tolerance may be linked to increased virulence. Although this situation has implications regarding the use of organic acids, it may only apply to circumstances in which reduced acid levels have induced resistance and virulence mechanisms in exposed organisms. Evaluating effectiveness of organic acids for specific applications requires more understanding general and specific stress response capabilities of foodborne pathogens. Development and application of molecular tools to study pathogen behavior in preharvest and postharvest food production environments will enable dissection of specific bacterial genetic regulation involved in response to organic acids. This could lead to the development of more targeted strategies to control foodborne pathogens with organic acids.

Effects of nondigestible oligosaccharides on Salmonella enterica serovar Typhimurium and nonpathogenic Escherichia coli in the pig small intestine in vitro

An in vitro intestinal tissue model was developed for the investigation of bacterial association in the pig small intestine under different dietary regimes. In preliminary experiments, jejunal and ileal tissue was taken from Danish Landrace pigs fed standard diet and inoculated with either Salmonella or nonpathogenic Escherichia coli strains. Higher numbers of salmonellae associated with the ileal tissues, but the numbers did not reach significance. Hence, jejunal sections were inoculated with nonpathogenic E. coli and ileal sections were inoculated with salmonellae in the presence of mannose or commercial nondigestible oligosaccharides (NDO) at 2.5%. There was a significant decrease in E. coli associated with the jejunum in the presence of mannose (P < 0.05). Furthermore, in pigs fed a diet supplemented with commercial NDO at 4% there was a significant reduction in the numbers of E. coli in jejunal organ cultures of pigs fed the FOS diet (P < 0.05). There was a reduction, though not a significant one, in the association of Salmonella sp. to the ileal sections of pigs fed the commercial FOS diet. The feeding of commercial GOS or its addition to organ cultures did not affect E. coli or Salmonella numbers.